The hematopoietic system is an important dose-limiting normal tissue for both chemotherapy and radiation therapy. It is the purpoe of these investigations to understand how the hematopoietic stem cell compartment functions and how it responds to these therapeutic agents. Related investigation concerns methods of improving bone marrow transplantation. SPECIFIC OBJECTIVES: Further investigation of the limited proliferative capacity of the stem cell compartment and how do cytotoxic agents affect this? Is the failure of serial transfer due to stem cell or stromal factors? Is the proposed model of the stem cell compartment valid? Can we separate the stem cells most important for cell renewal from those cells responsible for graft versus host disease? The studies will utilize bone marrow grown in vitro and assayed by its self-renewal capacity in vivo.